If you just borrowed a car, I don't understand what you are worried about. If there is more to this story, you should be discussing it with your bankruptcy attorney. If the vehicle was sold to you under this bill of sale, you aren't just borrowing it, you own it, even if there isn't a title. I suggest you keep your story straight when you file your bankruptcy because the way you have explained things, it sounds like you are not being truthful. Hope this perspective helps!
Facts are confused here so I am assuming you bought the car and have a bill of sale but no title. If that is true: (1) you must list ownership in your bankruptcy (2) the other party can not stop the bankruptcy but if he claims to still own the car he will have to make a claim in the bankruptcy court (3) if the car has any value over $3,000 the bankruptcy Trustee will take action to get the bill of sale so he can sell it; The bankruptcy absolutely stops anyone from trying to collect a debt by any means, so the other guy can not put a lien against your house.
This comment does not create an attorney-client relationship. The law and its application by the courts is constantly evolving and changing. This discussion is not to be taken as a definitive guide, and should not be relied upon to determine all fact situations. Each set of facts must be examined separately with the current case and statutory law analyzed and applied accordingly.